There are very few wedding rules that apply across all cultures. One of those rules is that nobody should try to upstage the bride. Mothers of the groom have tried it before, wearing "ivory" to weddings where the bride is wearing white while aggressively trying to justify it. I feel like women who do things like this must delusionally think that the attention they're getting for it is positive, but everybody who comments on a mere citizen wearing white at a wedding does it out of disdain and disgust.
It's one thing for a weird older relative to pull focus at your wedding, but it's entirely more nefarious for your own sister to try and pull focus as part of your bridal party. The sister of a bride who is insecure about her looks is trying to defend her choice to wear a flashy bridesmaid outfit, and people are ripping her to shreds for it.